1. Field of the Invention
The present invention relates to an electrochromic display device.
2. Description of Related Art
The publications in the forms of electronic books, that is, electronic publishing, have come to be actively performed in place of the publications by conventional printing techniques as electronic information networks have spread. As the apparatus displaying electronic information to be distributed in these networks, for example, cathode ray tube (CRT) displays and back light type liquid crystal displays have been used. However, the displays by means of these displays are restricted in places to be read, and are inferior also in the handling aspects of the displays in terms of their weights, sizes, shapes, and portability in comparison with the common displays printed on paper. Moreover, because these displays consume much electric power, the restriction of display times is also caused in the case of battery drives. Furthermore, all of these displays is a light emitting type display, and has the problem of causing extreme fatigue at the time of a long hour steady gaze.
Consequently, a display device capable of settling the problems mentioned above is desired, and further a rewritable display device is desired. As these display devices, a display called a paper-like display or electronic paper has been proposed. To put it concretely, for example, the following display devices have been proposed in the past: the display device of a reflective liquid crystal system, the display device of an electrophoretic system, the display device of the system of rotating dichromatic particles in an electric field, and the display device of an electrochromic system (see, for example, Japanese Patent Application Laid-Open Publications No. 2007-178733, No. 2007-314721, and No. 2008-032911).
Incidentally, a display device of the electrochromic system (electrochromic display device) uses a display material such as an electrochromic composition containing a precursor of a dye, such as a leuco dye, which colors on the surface of an electrode, as an essential ingredient. Because the leuco dye has a high chromogenic property at low power and the possibility of being capable of displaying each color, the leuco dye is a superior material as an electrochromic material. Furthermore, because the leuco dye is versatile as a recording material for thermal recording and the like, and because the leuco dye can be easily procured and supplied at a comparatively low cost, the leuco dye is a superior material as an electrochromic material also from the point of view of cost.
Accordingly, when the electrochromic display device using the leuco dye is applied as the electronic paper, it is desired to use a low cost drive system also as the drive system of the electrochromic display device, and, for example, it is desirable to use a passive matrix drive capable of driving the electrochromic display device without using any expensive materials, such as thin film transistors (TFTs).
However, when the electrochromic display device using the leuco dye is used as the display device of the passive matrix drive, then a sufficient display density cannot be obtained when the electrochromic display device is driven at a high speed, and the writing speed of several hundreds of milliseconds per line is required in order to obtain a high quality display. That is, for example, when the electrochromic display device using the leuco dye is applied as a sheet of electronic paper (A6 size), then 24 seconds (=0.1 second×240 lines) are estimated to be needed for displaying a page from a calculation based on the writing speed of 100 milliseconds per line even in the case of setting the resolution of the page to Quarter Video Graphics Array (QVGA), which is comparatively coarse, and the electrochromic display device has the problem of lacking utility.